The invention relates to drained-cathode cells for the electrowinning of aluminium from alumina, of the type comprising a series of anodes spaced by a sloped inter-electrode gap from one or more facing cathodes and arranged so the electrolyte circulates upwardly in the sloped inter-electrode gap assisted by anodically produced gases. The invention also relates to a method of producing aluminium in such cells as well as to cathodes and anodes designed for such cells.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
U.S. Pat. No. 3,400,061 (Lewis/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Traugott) disclose aluminium electrowinning cells with sloped drained cathodes and facing anodes sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the center of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
In U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and reimmersion of the assembly during operation, such assembly also operating with a drained cathode.
U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved using a tubular anode with suitable openings.
U.S. Pat. No. 5,651,874 (de Nora/Sekhar) proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. This publication discloses slurry-applied applications and novel drained cathode configurations, including designs where a cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom.
U.S. Pat. No. 5,683,559 (de Nora) proposed a new cathode design for a drained cathode, where grooves or recesses were incorporated in the surface of blocks forming the cathode surface in order to channel the drained product aluminium.
Recently it has become possible to coat carbon cathodes with a slurry which adheres to the carbon and becomes aluminium-wettable and very hard when the temperature reaches 700-800xc2x0 C. or even 950-1000xc2x0 C., as mentioned above. Though application of these coatings to drained cathode cells has been proposed, so far the commercial-scale application of this technology has been confined to coating carbon bottoms of cells operating with the conventional deep pool of aluminium. Further design modifications in the cell construction could lead to obtaining more of the potential advantages of these coatings.
While the foregoing references indicate continued efforts to improve cell operations, none suggests the invention and there have been no acceptable proposals for improving the cell efficiency, and at the same time facilitating the implementation of a drained cathode configuration with improved electrolyte circulation.
An object of the invention is to overcome problems inherent in the conventional design of cells used in the electrowinning of aluminium from alumina dissolved in molten fluoride-containing melts in particular cryolite, notably by proposing a drained cathode configuration incorporating an improved electrode arrangement.
Another object of the invention is to permit more efficient cell operation by modifying the design of the drained cathode(s) and/or of the anodes to improve the electrolyte circulation.
Yet another object of the invention is to provide an arrangement wherein gas release at a sloping anode surface is used to induce electrolyte circulation which in turn is facilitated by a novel cathode and/or anode design.
A further object of the invention is to provide a cell with a cathode of novel design enabling drained cathode operation where efficient electrolyte circulation is combined with ease of removal of the anodically produced gases and with ease of collection of the product aluminium.
A yet further object of the invention is to enhance the efficiency of electrolysis by supplying alumina to a circulating electrolyte to compensate for depletion during electrolysis, this electrolyte circulation being produced by means of a novel electrode configuration.
One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises one or more anodes and one or more cathodes. The or each anode and cathode respectively have a sloped V-shaped active anode surface and parallel sloped inverted V-shaped drained cathode surface facing one another and spaced apart by two sloped inter-electrode gaps, arranged so that the electrolyte circulates upwardly in the sloped inter-electrode gaps assisted by anodically produced gas and then returns from a top part to a bottom part of each inter-electrode gap along an electrolyte path. Each electrolyte path extends through vertical and horizontal passages as follows: for the cathode, a vertical passage from a top to a lower part of a cathode and then a horizontal passage in or under the lower part of the cathode; and/or for the anode, a horizontal passage in or on an upper part of an anode and then a vertical passage extending from the upper to a bottom part of the anode. Each horizontal passage extends substantially over the entire horizontal length of a corresponding inter-electrode gap.
In this context, a xe2x80x9cV-shaped surfacexe2x80x9d means a surface having a perpendicular cross-section which strictly or generally forms a V, in particular a flattened and/or truncated V. Such a surface may be generally conical, frusto-conical or bi-planar.
The drained-cathode cell according to the invention and the corresponding method of electrowinning aluminium have numerous advantages, including the following
a) The anodically produced gases are rapidly removed due to the slope of the anodic active surfaces.
b) The cell can be operated at high current density, providing for a sufficient upward removal of anodically produced gas to produce a corresponding upward circulation of the electrolyte in the anode-cathode gap.
c) The slope of the cathodic surfaces is sufficient to allow for efficient draining of the product aluminium, despite the counter-current of electrolyte entrained upwardly by the gas release.
d) The generally horizontal passage provides part of a return path for the electrolyte, enabling a steady-state circulation of the electrolyte around the electrodes.
e) An improved electrolyte circulation may be achieved by providing a plurality of return paths associated with both anodic and cathodic electrodes.
e) The induced electrolyte circulation can advantageously be combined with a supply of alumina to compensate for depletion during electrolysis. This supply of alumina may be adjacent to the upper end of the sloping inter-electrode gap or possibly over the anodes.
f) The cathode(s) can easily be made from the usual grades of carbon used for cathode applications, the sloping cathodic surfaces at least being coated with a suitable coating of aluminium-wettable refractory material, for example a slurry-applied coating containing titanium diboride, for example as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar) or WO 98/17842 (Sekhar/Duruz/Liu).
g) Making the cathodes with generally conical, wedge-shaped or prismatic recesses in the cathodic top face leads to a very compact and energy-efficient design.
h) The cells can be used with consumable carbon anodes, but great advantages can be secured by using substantially dimensionally-stable non-carbon oxygen evolving anodes, particularly in conjunction with cathodes having generally conical, wedge-shaped or prismatic recesses in its/their top face.
i) The cathodes can be suspended from the anodes, for ease of removal and reinsertion in the cell.
Each horizontal passage of the electrolyte path may be formed by an aperture extending through a cathode or an anode.
The or each cathode may be associated with an electrolyte path. The electrolyte path may extend through a vertical passage in the middle of an inverted V-shaped cathode surface from the top to the lower part of the or each cathode. Alternatively, the electrolyte path may extend through a vertical passage extending from adjacent a top part of a V-shaped cathode surface to the lower part of the or each cathode.
Similarly, the or each anode may be associated with an electrolyte path. The electrolyte path may extend through a vertical passage from the upper to the bottom part of the or each anode in the middle of a V-shaped anode surface. Alternatively, the electrolyte path may extend through a vertical passage from the upper part of an inverted V-shaped anode surface to adjacent a bottom part of the anode.
The horizontal passages may be delimited by an external upper face of an anode or an external lower face of a cathode.
The sloped drained cathode surfaces may lead down into an arrangement for collecting product aluminium.
The or each cathode may be connected to at least one anode by connection means made of materials of high electrical, chemical and mechanical resistance maintaining the inter-electrode gaps substantially constant, such that the or each cathode is removable and insertable into the cell with the anode(s) to which it is connected. The or each cathode may be thus suspended from at least one anode, or suspended from an anode by other means. Alternatively, the or each cathode may be mechanically secured between a pair of adjacent anodes by at least one horizontal electrically non-conductive bar or rod which is secured in the pair of adjacent anodes and which extends though the cathode. The electrically non-conductive bar or rod can extend through a plurality of cathodes.
Usually, the drained cathode surfaces have an aluminium-wettable coating. Moreover, the drained cathode surfaces can be made dimensionally stable by a slurry-applied coating of aluminium-wettable refractory material.
The fluoride-containing molten electrolyte of the cell can be essentially cryolite or cryolite with an excess of AlF3, typically an excess corresponding to about 25 to 35 weight % of the electrolyte. An excess of AlF3 in the electrolyte reduces the melting point of the electrolyte and permits cell operation with an electrolyte at lower temperature, typically from 780xc2x0 to 880xc2x0 C., in particular from 820xc2x0 to 860xc2x0 C.
The invention also relates to a method of producing aluminium in a cell as described above which contains dissolved alumina in a molten electrolyte. The method comprises: electrolysing dissolved alumina in the inter-electrode gaps, thereby producing aluminium on the drained cathode surface(s) and gas on the active anode surface(s). The electrolyte circulation upwardly in the sloped inter-electrode gaps is assisted by the upward removal of anodically produced gas. The electrolyte is returned from a top part to a bottom part of the inter-electrode gaps along the electrolyte paths. Alumina-depleted electrolyte is replenished with alumina in the electrolyte paths, preferably adjacent to the top parts of the inter-electrode gaps.
When the anodes are made of carbon material, CO2 is anodically produced during electrolysis.
Alternatively, the anodes are made of non-carbon inert material, preferably of metal or metal-based material, as for example disclosed in WO 99/36593, WO 99/36594, WO 00/06801, WO 00/06805 and WO 00/40783 (all in the name of de Nora/Duruz), U.S. Pat. No. 6,077,415 (Duruz/de Nora), WO 99/36591 and U.S. Pat. No. 6,103,090 (both in the name of de Nora). In a preferred embodiment, the anodes are made from a nickel-iron based alloy, e.g. as disclosed in WO 00/06803 (Duruz/de Nora/Crottaz) or WO 00/06804 (Crottaz/Duruz). When the anodes are made of inert material, oxygen is anodically evolved either by oxidising oxygen-containing ions directly on the active surfaces, or by firstly oxidising fluorine-containing ions that subsequently react with oxygen-containing ions, as described in PCT/IB99/01976 (Duruz/de Nora).
When the cell is operated with metal-based anodes, the molten electrolyte is advantageously substantially saturated with alumina, particularly on the electrochemically active anode surface, and with species of at least one major metal present at the surface of the anodes to maintain the anodes dimensionally stable, as disclosed in WO 00/06802 (Duruz/de Nora/Crottaz).
A xe2x80x9cmajor metalxe2x80x9d refers to a metal which is present at the surface of the anode, in particular in one or more oxide compounds, in an amount of at least 25% of the total amount of metal present at the surface of the anode.
The or each anode can be associated with an electrolyte path, alumina being fed from above the upper part of the or each anode where it is dissolved in the electrolyte and circulated along the electrolyte path to a lower part of the inter-electrode gap. Alternatively or cumulatively, the or each cathode can be associated with a electrolyte path, alumina being fed from above the top part of the or each cathode where it is dissolved in the electrolyte and circulated along the electrolyte path to a lower part of the inter-electrode gap.
Another aspect of the invention relates to a cathode of a cell for the electrowinning of aluminium from alumina dissolved in a molten fluoride-containing electrolyte as described above. The cathode comprises one or more inverted V-shaped sloped drained cathode surfaces facing during use one or more anodes and spaced therefrom by inter-electrode gaps. The cathode is associated with one or more electrolyte paths for the return of electrolyte from a top part to a bottom part of the inter-electrode gaps. The or each electrolyte path extends through a vertical passage from a top to a lower part of the cathode and then through a horizontal passage in or under the lower part of the cathode. The or each horizontal passage extends substantially over the entire horizontal length of the corresponding inverted V-shaped cathode surface.
A further aspect of the invention relates to an anode of a cell for the electrowinning of aluminium from alumina dissolved in a molten fluoride-containing electrolyte as described above. The anode comprises a V-shaped sloped active anode surface facing during use a correspondingly sloped drained cathode surface and spaced therefrom by inter-electrode gaps. The anode is associated with an electrolyte path for the return of electrolyte from a top part to a bottom part of the inter-electrode gaps. The electrolyte path extends through a horizontal passage in or on an upper part of the anode and then through a vertical passage extending from the upper part to a bottom part the anode. The horizontal passage extends substantially over the entire horizontal length of the V-shaped anode surface.
The invention relates also to a drained-cathode cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises a series of anodes suspended over one or more cathodes. The anodes and the cathode(s) respectively have sloped active anode surfaces and parallel sloped drained cathode surfaces facing one another and spaced apart by sloped inter-electrode gaps, arranged so the electrolyte circulates upwardly in the sloped inter-electrode gaps assisted by anodically produced gas, and then returns from top parts to bottom parts of the inter-electrode gaps along electrolyte paths. Each electrolyte path extends through horizontal and vertical passages as follows: a vertical passage associated with a cathode and then a horizontal passage in or under a lower part of the cathode, and/or a horizontal passage in or on an upper part of an anode and then a vertical passage associated with the anode. Each horizontal passage extends substantially over the entire horizontal length of a corresponding inter-electrode gap.